Speed responsive motor starting system

ABSTRACT

A semiconductor switching circuit responsive to the speed of a single phase induction motor for controlling the starting operation thereof. A power bilateral semiconductor switching device is connected in series with the motor starting reactance for controlling current flow therethrough. A logic bilateral semiconductor switching device is coupled to the gate electrode of the power device for enabling and disabling the operation thereof. A current sensing transformer is coupled in circuit with the motor run winding for controlling the logic switching device in accordance with the motor speed. A capacitor is connected in series in the gate electrode circuit for the power switching device for advancing the phase angle of the gating current supplied thereto.

United States Patent Fink, Jr. et al.

[54] SPEED RESPONSIVE MOTOR STARTING SYSTEM [72] Inventors: Leon Fink,Jr., Arlington; David C.

Fricker, Hurst, both of Tex.

[73] Assignee: ECC Corporation, Euless, Tex.

[22] Filed: Mar. 12, 1971 [2]] Appl. No.: 123,735

52 User... [51] Int. Cl. .......H02p1/44 [58], Field of Search ..318/220R, 221 R, 221 E, 227,

[56] References Cited UNITED STATES PATENTS 3,376,484 4/1968 Lewus..318/221 E 3,414,789 -l2/l968 Prouty ....3l8/221 E 3,544,869 l2/l970Plouffe et al ..318/221 R [451- Apr. 18, 1972 Primary Examiner-Gene Z.Rubinson Attorney-Giles C. Clegg, Jr., Richard E. Bee and Jack A.

Kanz

[57] ABSTRACT A semiconductor switching circuit responsive to the speedof a single phase induction motor for controlling the starting operationthereof. A power bilateral semiconductor switching device is connectedin series with the motor starting reactance for controlling current flowtherethrough. A logic bilateral semiconductor switching device iscoupled to the gate electrode of the power device for enabling anddisabling the operation thereof. A current sensing transformer iscoupled in circuit with the motor run winding for controlling the logicswitching device in accordance with the motor speed. A capacitor isconnected in series in the gate electrode circuit for the powerswitching device for advancing the phase angle of the gating currentsupplied thereto.

10 Claims, 2 Drawing Figures VLINE PATENTEUAPR 18 m2 3, 657, 621

LEON FINK,JR. DAVID C FRICKER INVENTORS ATTORNEYS START 1 SPEEDRESPONSIVE MOTOR STARTING SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates to solid state or semiconductor motor startingcircuits and, more particularly, to semiconductor motor startingcircuits especially adapted for use in the starting of single phaseinduction motors.

Various semiconductor motor starting circuits have been heretoforeproposed. In a typical such circuit, a bilateral semiconductor switchingdevice is connected in series with the starting reactance of the motorand a current sensing resistor is connected in series with the runwinding of the motor. The current sensing resistor is connected to thegate electrode of the semiconductor switching device for controllingsame for enabling line current to flow through the starting reactancewhen the motor speed is below a prescribed minimum value.

Circuits of this proposed type suffer from various disadvantages. Forone thing, the alternating-current voltage appearing across the currentsensing resistor will usually be lagging in phase relative to thealternating current flowing through the starting reactance and switchingdevice. As a result, the switching device will not immediately commenceconduction at the beginning of each half cycle of the starting reactancecurrent. Instead, there will be a time lag caused by the lag intriggering of the switching device. Consequently, the current flowthrough the starting reactance will have a waveform characterized by azero level notch immediately following the beginning of each half cycle.The presence of such notches results in a decrease in the effectivepower supplied to the starting reactance, thus reducing the startingtorque applied to the motor.

Another disadvantage of motor starting circuits of the foregoing type isthat they are sensitive to variations in the value of thealternating-current line voltage. In particular, variations in the linevoltage produce corresponding variations in the time lag in thetriggering of the switching device. The resulting effect on the motorstarting torque limits the range of line voltage values over which themotor may be reliably used.

It is an object of the invention, therefore, to provide a new andimproved speed responsive motor starting system which enables a fullerrealization of the maximum starting torque capability of the motor.

It is another object of the invention to provide a new and improvedspeed responsive motor starting system which has a greater tolerance forvariations in the value of the alternating current line voltage.

For a better understanding of the present invention, together with otherand further objects and features thereof, reference is had to thefollowing description taken in connection with the accompanying drawing,the scope of the invention being pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWING Referring to the drawing:

FIG. 1 is a schematic circuit diagram of a motor starting systemconstructed in accordance with the present invention; and

FIG. 2 is a vector diagram used in explaining the operation of the FIG.1 system.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Referring to FIG. 1, there isshown the major electrical parts of a single-phase alternating-currentinduction motor, such parts including a run winding and a startmechanism represented by a start winding 11. Run winding 10 and startwinding 11 are located in parallel circuit branches which are connectedbetween a pair of supply circuit means adapted to be individuallyconnected to different sides of an electrical power source. A first ofthese supply circuit means includes a power supply conductor 12 having apower supply terminal 13 at one end thereof. The second of these supplycircuit means includes a second power supply conductor 14 having a powersupply terminal 15 at one end thereof. In use, terminals 13 and 15 areconnected to different sides of an electrical power source such as, forexample, a two-wire alternating-current power line. The startingreactance (represented by start winding 11) need not be purely aninductance but may instead take various other forms as are well known inthe induction motor 311.

Connected in series with the start winding 11 is a first gate controlledbilateral semi-conductor switching device 16 having a pair of powerelectrodes 17 and 18 and a control or gate electrode 19. For sake ofconvenience, the power electrode 18 located on the same side of thedevice 16 as the gate electrode 19 will be referred to as the cathode,while the other power electrode 17 will be referred to as the anode.Also, the device 16 will sometimes be referred to as the power switchingdevice. The switching device 16 is connected in series with the startmechanism 11 by means of its power electrodes 17 and 18, the anode 17being connected to the start mechanism 11 and the cathode 18 beingconnected to the lower power supply conductor 14.

The operation of the power switching device 16 is controlled by a secondgate controlled bilateral semiconductor switching device 20 having apair of power electrodes 21 and 22 and a control element represented bya control electrode or gate electrode 23. For sake of convenience, thepower electrode 22 located on the same side of the device 20 as the gateelectrode 23 will be referred to as the cathode, while the other powerelectrode 21 willbe referred to as the anode. Also, the device 20 willsometimes be referred to as the logic switching device. This secondswitching device 20 is connected to supply gating current to the gateelectrode 19 of the power switching device 16. More particularly, theanode 21 is connected by way of a capacitor 24 and a resistor 25 to theupper power supply conductor 12, while the cathode 22 is connecteddirectly to the gate electrode 19 of the power switching device 16 bymeans of conductor 26.

Operation of the logic switching device 20 is controlled by currentsensing means represented by a current transformer 27. Currenttransformer 27 includes a first or primary winding 28 connected inseries with the motor run winding 10 and a second or output winding 29coupled to the gate electrode 23 of the logic switching device 20. Theupper end of the secondary or output winding 29 is connected to the gateelectrode 23 by way of a current limiting and trimming resistor 30,while the lower end of winding 29 is connected to the lower power supplyconductor 14. As indicated by the dots on the windings of thetransformer 27, the output winding 29 is coupled to the gate electrode23 in an opposite polarity manner relative to the primary winding 28such that an additional phase shift is added to the trode 23. In otherwords, when the upper end of the primary winding 28 is positive, theupper end of the secondary winding- 29 is negative and vice versa.Transformer 27 is constructed so that the impedance of the primarywinding 28 is very, very small.

Considering how the operation of the FIG. 1 embodiment, the magnitude ofthe current I,.,,,, flowing through the motor run winding 10 variesinversely with the speed of rotation of the motor. When the rotor of themotor is stationary, the cur-' rent flow is fairly heavy. As the motorpicks up speed, the current flow decreases. Because of the inductivenature of the run winding10, the current I lags the line voltage Vapplied between the power supply conductors l2 and 14. A typical phaserelationship for these quantities is indicated in the vector diagram ofFIG. 2. Current transformer 27 acts to sense the magnitude of thecurrent flowing through the run winding 10 and to develop across thesecondary or output winding 29 thereof an alternating-current voltage V,which is proportional in magnitude to the magnitude of the run windingcurrent I,.,,,, The secondary voltage produces the gating current gatingcurrent supplied to the gate elec-' the logic device 20 is shifted inphase by a factor of 180 with respect to the run winding current flowingthrough the transformer primary winding 28. Thus, for the exampledepicted in FIG. 2, if the run winding current lags the line voltage by70, then the secondary voltage V, leads the line voltage by a factor ofl l.

When the gating current supplied to the gate electrode 23 exceeds apredetermined trigger level set by the internal characteristics of thelogic device 20, the logic device 20 is conditioned to conduct currentfrom one power electrode to theother whenever the voltage across thepower electrodes exceeds a certain minimum value. Such power electrodecurrent may flow from the anode 21 to the cathode 22 or vice versa,depending on the polarity of the line voltage at the moment considered.When in a conductive condition, the impedance between power electrodes21 and 22 is of a very low value. When in a nonconductive condition, theimpedance between the power electrodes 21 and 22 is relatively high. Thedevice 20 will switch to the conductive mode for either positive ornegative polarity gating current at the gate electrode 23, provided theamplitude of such current is above the trigger level.

Assuming for the moment that the motor has just been turned on and thatthe rotor is still stationary, then both the run winding current and thesecondary voltage V, are relatively large and the switching device 20 isrendered conductive at some point during each half cycle of thesecondary voltage V,. With the device 20 conductive, current flows fromthe upper power supply conductor 12, through the resistor 25, thecapacitor 24, the switching device 20 and the gate electrode 19 tocathode 18 portion of the power switching device 16, or vice versa. Thiscurrent I, is the gating current for the power device 16 and is ofsufficient magnitude to render the power device 16 conductive at somepoint during each half cycle. The use of the capacitor 24 in thiscircuit causes the gating current I, to lead the line voltage by somefactor such as, for example, 40. This results from the fact that thevoltage drop across the capacitor 24, namely V,, lags the line voltage,hence causing the voltage drops across, the resistive elements in thecircuit to lead the line voltage in order that the vector sum of suchcapacitive and resistive drops may add up to the value of the linevoltage, both in amplitude and phase. Since the gating current I, is inphase with the resistive drops, such gating current leads the linevoltage.

The operation of the logic switching device 20 is such that it turns offeach time the current I, flowing between its power electrodes 21 and 22crosses the zero amplitude axis. Since the secondary voltage V, suppliedto the gate electrode 23 leads the current I, flowing between the powerelectrodes 21 and 22, the device 20 will have a relatively high value ofgate electrode drive current at the moment of occurrence of the zeroaxis crossing of the power electrode current 1,. Consequently, the logicdevice 20 will turn on again at practically the same moment that thecurrent I, crosses the zero axis. Thus, provided the secondary voltageV, is above the trigger level, the device 20 will be conductivepractically the entire time.

The flow of gating current I, serves to render the power switchingdevice 16 conductive. The power device 16 works in the same manner asthe logic device 20. Thus, since the gating current I, leads the currentI,,,,, flowing through the start winding 11, the power device 16 hasrelatively high values of gating current I, at its gate electrode 19 atthe same moments that the start current 1 makes its zero axis crossings.Consequently, the power device 16 is turned on again practically asquick as it is turned off by such zero axis crossings. Thus, the powerdevice 16 is conductive very nearly 100 per cent of the time. Thissubstantially eliminates the undesired notch effect in the start wingingcurrent, thus allowing the motor to more nearly realize its maximumstarting torque capability.

So long as the run winding current'exceeds a threshold valuecorresponding to the trigger level for the logic device 20, both of theswitching devices 16 and 20 remain operative. When the motor reaches thedesired minimum operating speed, the run winding current falls belowthis threshold value.

' This causes the secondary voltage V, to fall below the trigger levelfor the logic device which, in turn, renders the logic device 20inoperative. The resulting high impedance between the power electrodes21 and 22 reduces the current I, to a negligible value which, in turn,renders the power switching device 16 imperative. The resulting highimpedance between power electrodes 17 and 18 reduces the current flowthrough variations in the magnitude of the altemating-current linevoltage applied to the power supply terminals 13 and 15. This enablesthe motor to be used satisfactorily underfairly adverse line voltageconditions. Also, because of the very low primary impedance of thecurrent sensing transformer 27, the power dissipation in the primarywinding 28 is very small.

While there has been described what is at present considered to be apreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,intended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. A speed responsive motor starting system comprising:

a motor including a run winding and a start mechanism;

a first and bilateral switching device having a pair of power electrodesand a control electrode;

a second switching device having a pair of power electrodes and acontrol element;

first circuit means connecting the first switching device by its powerelectrodes in series with the motor start mechanism;

second circuit means connecting the second switching device by its powerelectrodes to supply gating current to the control electrode of thefirst switching device;

current sensing means responsive to the current flowing through the runwinding for supplying gating current to the control element of thesecond switching device;

and capacitor means connected in series in the second circuit means foradvancing the phase angle of the gating current supplied to the controlelectrode of the first switching device for minimizing the notch effectin the current flowing through the start mechanism.

2. A speed responsive motor starting system in accordance with claim 1wherein the second switching device is also a bilateral switching deviceand its control element is a control electrode.

3. A speed responsive motor starting system in accordance with claim 2wherein both the first and'the second switching devices are gatecontrolled bilateral semiconductor devices.

4. A speed responsive motor starting system in accordance with claim 2wherein the current sensing means comprises transformer means responsiveto current flowing through the motor run winding and having an outputwinding for supplying gating current to the control electrode of thesecond switching evice.

5. A speed responsive motor starting system in accordance with claim 4wherein the output winding of the transformer means is coupled to thecontrol electrode of the second 1 switching device with a polarity suchthat an additional l80 phase shift is added to the. gating currentsupplied to such control electrode.

6. Aspeed responsive motor starting system in accordance with claim 2wherein the current sensing means comprises a additional 180 phase shiftis added to the gating current supplied to such control electrode.

8. A speed responsive motor starting system in accordance with claim 4wherein the system includes a pair of supply circuitmeans adapted to beindividually connected to different sides of an electrical power source,wherein the motor start mechanism and the first switching device areconnected in series between the two supply circuit means, wherein onepower electrode of the second switching device is coupled to one of thesupply circuit means and the other power electrode of the secondswitching device is coupled to the control electrode of the firstswitching device and wherein the capacitor means is connected in serieswith the power electrodes of the second switching device intermediatethe one supply circuit means and the control electrode of the firstswitching device.

9. A speed responsive motor starting system in accordance with claim 8wherein the transformer means includes a transformer having a firstwinding connected in series with the motor run winding and'a secondwinding coupled to the control electrode of the second switching device.

10. A speed responsive motor starting system in accordance with claim 9wherein the second transformer winding is coupled to the controlelectrode of the second switching device in an opposite polarity mannerrelative to the first transformer winding so that an additional 180phase shift is added to the gating current supplied to the controlelectrode of the second switching device.

1. A speed responsive motor starting system comprising: a motorincluding a run winding and a start mechanism; a first and bilateralswitching device having a pair of power electrodes and a controlelectrode; a second switching device having a pair of power electrodEsand a control element; first circuit means connecting the firstswitching device by its power electrodes in series with the motor startmechanism; second circuit means connecting the second switching deviceby its power electrodes to supply gating current to the controlelectrode of the first switching device; current sensing meansresponsive to the current flowing through the run winding for supplyinggating current to the control element of the second switching device;and capacitor means connected in series in the second circuit means foradvancing the phase angle of the gating current supplied to the controlelectrode of the first switching device for minimizing the notch effectin the current flowing through the start mechanism.
 2. A speedresponsive motor starting system in accordance with claim 1 wherein thesecond switching device is also a bilateral switching device and itscontrol element is a control electrode.
 3. A speed responsive motorstarting system in accordance with claim 2 wherein both the first andthe second switching devices are gate controlled bilateral semiconductordevices.
 4. A speed responsive motor starting system in accordance withclaim 2 wherein the current sensing means comprises transformer meansresponsive to current flowing through the motor run winding and havingan output winding for supplying gating current to the control electrodeof the second switching device.
 5. A speed responsive motor startingsystem in accordance with claim 4 wherein the output winding of thetransformer means is coupled to the control electrode of the secondswitching device with a polarity such that an additional 180* phaseshift is added to the gating current supplied to such control electrode.6. A speed responsive motor starting system in accordance with claim 2wherein the current sensing means comprises a transformer having a firstwinding connected in series with the motor run winding and a secondwinding coupled to the control electrode of the second switching device.7. A speed responsive motor starting system in accordance with claim 6wherein the second winding is coupled to the control electrode of thesecond switching device in an opposite polarity manner relative to thefirst winding so that an additional 180* phase shift is added to thegating current supplied to such control electrode.
 8. A speed responsivemotor starting system in accordance with claim 4 wherein the systemincludes a pair of supply circuit means adapted to be individuallyconnected to different sides of an electrical power source, wherein themotor start mechanism and the first switching device are connected inseries between the two supply circuit means, wherein one power electrodeof the second switching device is coupled to one of the supply circuitmeans and the other power electrode of the second switching device iscoupled to the control electrode of the first switching device andwherein the capacitor means is connected in series with the powerelectrodes of the second switching device intermediate the one supplycircuit means and the control electrode of the first switching device.9. A speed responsive motor starting system in accordance with claim 8wherein the transformer means includes a transformer having a firstwinding connected in series with the motor run winding and a secondwinding coupled to the control electrode of the second switching device.10. A speed responsive motor starting system in accordance with claim 9wherein the second transformer winding is coupled to the controlelectrode of the second switching device in an opposite polarity mannerrelative to the first transformer winding so that an additional 180*phase shift is added to the gating current supplied to the controlelectrode of the second switching device.